1. Field of the Invention
The present invention relates to an optical module with a simple optical system structure and an optical pickup including the same.
2. Description of the Related Art
Optical pickups are used to record information on and/or reproduce information from recording media. Such optical pickups are composed of various optical devices that include a light source that emits a laser beam, an optical path changing device that changes the proceeding path of light emitted from a light source, an objective lens focusing the light emitted from the light source to form an optical spot on a recording surface of the recording medium, and a photodetector receiving light reflected from the recording surface of the recording medium to detect an information signal and/or an error signal.
FIG. 1 is a partial sectional perspective view of a semiconductor laser module used as a light source in an optical pickup. Referring to FIG. 1, a general semiconductor laser module includes a cap 3 having a glass window 5, through which a laser beam transmits, on a body 1. In addition, a mount 2 is disposed in the body 1, and a semiconductor laser 10 that emits a laser beam with a predetermined wavelength to both sides is installed on a side wall of the mount 2. Also, a monitoring photodetector 7 that receives a laser beam L2 emitted by the semiconductor laser 10 away from the glass window 5 is disposed in the body 1. The monitoring photodetector 7 monitors a laser beam L1 emitted by the light source 10 toward the glass window 5 based on the detected signal. The semiconductor laser 10 and the monitoring photodetector 7 are connected to a lead 9 used as a conductive path.
Referring to FIG. 2, the laser beam emitted from a predetermined position of the active layer 11 in the semiconductor laser 10 is divergent and has an elliptical cross section. Since sizes of emitting apertures through which the laser beams are emitted, in the thickness direction and the width direction crossing the thickness direction at a right angle of the active layer 11 are different from each other, the beams are diffracted differently from each other. Here, the laser beam propagating in the thickness direction (direction Y) is the same as that emitted from the front portion of the laser 10, and the laser beam in the width direction (direction X) is the same as that emitted from a point that is a distance ΔZ from a front portion of the semiconductor laser. In FIG. 2, θ∥ is a spread angle in the width direction, and θ⊥ is a spread angle in the thickness direction.
Astigmatism caused by structural characteristics of the above semiconductor laser module is not important, if the light is exactly focused on the optical disc.
However, if the light is not exactly focused on the optical disc due to a change of optical length in the optical system, the light is distorted in one direction. This causes degradation of recording and/or reproducing.
The astigmatism does not cause a problem when the optical system has perfect structure and the optical length of the optical system is stable at low or high temperatures. However, since optical elements constituting the optical pickup are adhered to a base using an ultraviolet (UV) bond and the optical pickup is operated at a high temperature when it performs recording and/or reproducing operations, the optical length of the optical system may change. The more optical elements constituting the optical pickup, the more variable the optical length of the optical system becomes. Therefore, the astigmatism in the semiconductor laser has a significant negative effect on the recording and/or reproducing performances of the optical pickup.
When the optical recording and/or reproducing device including the optical pickup is operated, a spindle motor that rotates the optical disc and various chips that process signals operates simultaneously. Because the elements generate heat when information is recorded on the optical disc or information recorded on the optical disc is read, an inner temperature of the optical pickup may rise to a high temperature.
For example, since the power of a laser beam in a recording operation of a digital versatile disc (DVD) is higher than that in a reproducing operation of the DVD, the optical pickup operates at a higher temperature when it performs the recording operation than during the reproducing operation. In the recording operation, the heat is generated by a chip used to process signals and the semiconductor laser module used as the light source, thus increasing the inner temperature of the optical recording and/or reproducing device to about 70° C. Even if the optical recording and/or reproducing device for the DVD includes a cooling system such as a fan, the inner temperature rises higher than 60° C., because the optical recording/reproducing device for the DVD includes a chip generating large amounts of heat, for example, a chip for servo. Also, in the reproducing operation, the inner temperature may rise to about 45° C.
Therefore, the optical pickup always operates at a high temperature when the optical recording and/or reproducing device is operated. In a hot external environment or summer season, the temperature environment for using the optical pickup become worse.
Operation at high temperatures may cause a distortion of the optical system length and the optical elements. That is, to adhere the optical elements to the base, a UV bond is generally used. However, the part adhered by the bond may be distorted or expanded at the high temperature. Thus, the optical elements may be distorted and thus the optical path may be slightly changed.
As described above, when the optical paths of the optical elements are changed, loss of light greatly increases and thus the amount of light propagated to the recording medium (not shown) is reduced. Consequently, signal reduction or non-operation of the optical pickup may occur when the light reflected from the recording medium is not received on the photodetector (not shown), and/or otherwise deviates from the photodetector.
Therefore, in the high temperature operation, the signal recording and/or reproducing ability of the optical pickup gets worse. Also, in the high temperature operation, the large number of optical elements constituting the optical pickup increase the possibility of non-operation of the optical pickup. Additionally, the large number of the optical elements constituting the optical pickup increase manufacturing costs of the optical pickup.
When the optical element is adhered to the base, tolerance is a design factor that must be considered. For example, when numerous optical elements are used as the recording optical pickup, even if the optical elements have low tolerance, the entire optical pickup has a larger tolerance. In more detail, positions of the optical elements are decided according to focal lengths of the lens and optical length design. Here, even though a sample or a product is fabricated according to a design, since the tolerance of the design and the fabrication tolerance of each optical element always exists, the sample or the product may not be exactly the same as the design. Moreover, when the optical elements are assembled on the base such that the tolerances of the base and the optical elements are added together, the overall tolerance increases.
Generally, the light that passes through a collimating lens, which is used to change the diverging laser beam emitted from the semiconductor laser used as the light source in an optical pickup into parallel light, may converge or diverge due to a difference of the optical path caused by the above assembling error.
Since aberration is large at an edge of a lens in comparison with the center of the lens, the optical system of the optical pickup includes an objective lens which has minimal spherical aberration when parallel light enters and a collimating lens. However, when light incident on the objective lens is convergent or divergent due to an assembling error, the aberration increases and the light focused on the recording medium has an asymmetric shape. When the asymmetric light is focused on the recording medium, the light received by the photodetector becomes asymmetric and thus jitter increases. Also, since the focal point of the light is not exactly positioned on the photodetector, signal sensitivity is lowered.
Therefore, points where optical elements are adhered by the UV bond should be reduced considering the structural characteristics of the semiconductor laser causing astigmatism, operation of the optical pickup at high temperature, manufacturing costs and assembling errors of the optical pickup.